1. Field of the Invention
The present invention relates to a slot line planar antenna, and more particularly to a two-element and a multi-element slot line antenna which are simple in configuration and have a plurality of arrayed slot line antenna elements that are excited in phase.
2. Description of the Related Arts
Planar antennas are widely used in, for example, radio communications and satellite broadcasting because of their characteristics of ease in machining, small size, light weight, and the like. Planar antennas are classified into a microstrip line type, a slot line type, and the like. Generally, the microstrip line planar antenna is often used since it has a simple feed system structure, good radiation characteristics, and the like.
However, the microstrip line planar antenna is disadvantageous in a narrow frequency band in which it can operate a relatively low antenna gain, and difficulties in suppressing orthogonal components from antenna elements and feed system. From the foregoing, the slot line planar antenna has gained the spotlight because it has wide band frequency characteristics and less radiations of orthogonal components from antenna elements themselves, as compared with the microstrip line planar antenna.
Exemplary configurations of conventional slot line planar antennas will be described with reference to FIGS. 1A, 1B, 2A, 2B, 3A, 3B, 4A, 4B and 5. In the accompanying drawings, dotted portions on plan views represent areas over which conductors are formed on a first main surface of a substrate. Solid black areas in cross-sectional views represent cross-sections of conductors.
Each of illustrated slot line planar antennas comprises a substrate 1 made of a dielectric material or the like; antenna element 3 through which electromagnetic waves are transmitted or received, and feed systems 4a to 4c for transmission and reception disposed on substrate 1. Antenna element 3 is formed by routing a so-called slot line, which is a linear or loop-shaped open line, in conductor 2 formed on one main surface of substrate 1. A resonant frequency on an electric field plane depending on the length of the slot line is defined as a frequency at which electromagnetic waves are transmitted and received through antenna element 3. Generally, the length of the slot line is set to xcex/2, where xcex is the wavelength of the transmission and reception frequency.
In slot line antenna element 3, the slot line is a balanced transmission line, and an electric field plane is produced in a direction orthogonal to an electric field which is excited in the same direction alternately between the conductors on both sides of the open line, so that a complete standing wave of electromagnetic field is generated in the slot line. Generally, a cross section in the electric field direction, i.e., in the width direction of the open line is referred to as an electric field plane (E plane), and a cross section in the magnetic field direction, i.e., in the lengthwise direction of the open line is referred to as a magnetic field plane (H plane).
In the slot line planar antenna, since the slot line is formed on one main surface of substrate 1 on which conductor 2 is formed to produce the electromagnetic field plane on the same surface, less orthogonal components are produced in the electric field plane and magnetic field plane, as compared with a microstrip line planar antenna in which an electric field is excited between both main surfaces of a substrate.
In a slot line planar antenna, a microstrip line or a coplanar line, for example, is selected for a feed system when slot line antenna element 3 is linear. A slot line planar antenna illustrated in FIGS. 1A and 1B comprises linear slot line antenna element 3 and a feed system based on microstrip line 4a. Microstrip line 4a, which constitutes the feed system, is formed on the other main surface of substrate 1 such that it is in close proximity to one end of slot line antenna element 3 in the lengthwise direction, and extends in a direction orthogonal to the lengthwise direction of antenna element 3. Microstrip line 4a is electromagnetically coupled to slot line antenna element 3.
A slot line planar antenna illustrated in FIGS. 2A and 2B comprises a linear slot line antenna element 3 and a feed system based on coplanar line 4b. Coplanar line 4b is formed on one main surface of substrate 1 such that it extends from the center of slot line antenna element 3 in a direction orthogonal to the lengthwise direction of slot line antenna element 3.
When slot line antenna element 3 is formed in a loop shape as illustrated in FIGS. 3A and 3B, slot line 4c, for example, is selected for a feed system. In this event, feed slot line 4c is routed on one main surface of substrate 1, and is connected to one end side of a corner of the slot line which constitutes antenna element 3.
In the respective slot line planar antennas described above, electromagnetic waves, i.e., high frequency signals, are propagated from the feed system, which may be a microstrip line, a coplanar line or a slot line, to slot line antenna element 3 or from slot line antenna element 3 to the feed system upon transmission and reception.
Since these planar antennas each provide a low antenna gain with single slot line antenna element 3, a plurality of slot line antenna elements 3 are arranged to form, for example, an array antenna for improving the antenna gain. FIG. 4 illustrates an array antenna which comprises a plurality of linear slot line antenna elements 3, for example, arranged one-dimensionally on one side of microstrip line 4a which serves as a feed system. FIG. 5 illustrates an array antenna using loop-shaped slot line antenna elements 3 which are arranged on both sides of coplanar line 4b, which serves as a feed system, through short feed slot line. Pairs of slot line antenna elements 3 are arranged in the longitudinal direction of coplanar line 4b. 
However, either of the foregoing slot line array antennas has slot line antenna elements 3 simply arranged one-dimensionally along the feed system, which is made of the microstrip line or coplanar line, formed on one main surface of substrate 1. It is difficult to two-dimensionally arrange such antenna elements to form a multi-element array antenna.
When a two-dimensional array antenna is implemented by placing two or two pairs or more of slot line antenna elements 3 not only along the longitudinal direction of a feed system but also in a direction orthogonal to which the feed system extends, the feed system will interlace on one main surface of substrate 1, causing difficulties in providing a multi-element array antenna. In this event, even if the feed system drawn about to arrange antenna elements in two-dimensional directions, the respective slot line antenna elements will be fed from a feed end with different feed lengths from one another, thereby making it difficult to excite the respective slot line antenna elements in phase, with a resulting reduction in the directivity. Of course, the reduction in the directivity can be avoided if the feed system is set such that the antenna elements are matched in phase, in which case, however, complicated designing is imposed for the feed systems. In addition, when microstrip lines alone are used for the feed system in an array antenna, orthogonal components are necessarily produced and can be difficult to suppress.
While an existing two-dimensional multi-element antenna array employs a feed system comprised of tubular metal waveguides such as radial lines or the like, the use of the tubular waveguides results in a three-dimensional structure, inevitably causing a large sized antenna array.
As described above, a planar antenna having slot line antenna elements is practically limited to a one-dimensional array antenna in actuality due to a feed system based on a microstrip line, a coplanar line or a slot line. Therefore, a demand exists for realization of a practical two-dimensional multi-element array antenna in a planar circuit configuration. In the following description, the term xe2x80x9cslot line array antennaxe2x80x9d is given to a planar array antenna which comprises a plurality of slot line antenna elements arranged in a planar configuration.
It is an object of the present invention to provide a two-element slot line array antenna which is simple in configuration, exhibits a good directivity, and functions as a basic unit when a plurality of slot line antenna elements are two-dimensionally arranged to form a planar array antenna.
It is another object of the present invention to provide a multi-element slot line array antenna which is simple in configuration, and exhibits a good directivity.
A slot line array antenna according to the present invention comprises, as a basic configuration, a substrate having a first and a second main surface, a conductor disposed on the first main surface, a slot line formed in the conductor, a pair of slot line antenna elements formed in the conductor, and a feed system disposed on the substrate and having a feed end for feeding the pair of antenna elements. The feed system includes the slot line. In this basic configuration, the pair of antenna elements are arranged in parallel with an electric field plane or a magnetic field plane formed by the slot line. The pair of antenna elements and slot line are arranged in mirror symmetry with respect to a magnetic field plane or electric field plane which starts at the feed end, so that the pair of antenna elements are excited in phase.
The basic configuration is a two-element slot line array antenna which has a pair of slot line antenna elements arranged in mirror symmetry, including the feed system, for excitation in phase. The two-element slot line array antenna has the same feed length from the feed end to each antenna element, and therefore exhibits a good directional characteristic.
In the present invention, the basic configurations can be combined to form a multi-element slot line array antenna which has 4, 8, 16, or a larger number of antenna elements. Specifically, a pair of primary basic units, each of which is the two-element slot line array antenna described above, are arranged along an electric field plane or a magnetic field plane orthogonal to the direction in which the slot line antenna elements are arranged. The feed ends of the feed systems in the respective primary basic units are connected to each other to form a common feed line, and a feed line is routed to intersect with the common feed line at the midpoint thereof. A resulting four-element slot line array antenna has four antenna elements likewise arranged in mirror symmetry, and corresponds to a secondary basic unit.
In this way, a pair of feed end sides in the previous order basic units is connected to each other to form a common feed line, the previous order basic units are arranged along an electric field plane or a magnetic field plane orthogonal to the direction in which the second previous order basic units are arranged in the previous order basic units, and a feed line is routed to intersect with the common feed line at the midpoint thereof. A resulting multi-element slot line array antenna has a number of slot line antenna elements twice as much as those included in the previous order basic units. Since the slot line antenna elements are arranged in mirror symmetry with respect to the electric field plane or magnetic field plane starting at the feed end likewise in such a multi-element slot line array antenna, the array antenna has the same feed length from the feed end to each slot line antenna element. Consequently, the array antenna is free from a phase difference between the respective antenna elements, and exhibits a good directional characteristic.
In the multi-element slot line array antenna according to the present invention, feed lines, which make up the feed system, are routed independently of each other, and electromagnetically coupled to each other between both main surfaces of the substrate, thereby preventing the feed lines from interlacing on the same main surface of the substrate. Thus, the present invention can create a compact multi-element array antenna using slot line antenna elements in a planar structure, rather than a three-dimensional structure, in a simple configuration.
The present invention also provides a variety of two-element slot line array antennas, each of which oscillates in phase. Also, particularly, in the present invention, a feed line in a slot line structure is provided on a first main surface of a substrate, and a feed line in a microstrip line structure is provided on a second main surface of the substrate, as a feed system, to actively utilize electromagnetic coupling between the slot line and microstrip line, a function of series in-phase branch or a series in-phase combination therebetween, and a function of parallel anti-phase branch or parallel anti-phase combination therebetween.